1. Field of the Invention
The present invention relates to an optical amplifier that optically amplifies signal light by pump light, using an optical waveguide path to which a fluorescent material is doped, and to an optical transmission system using such optical amplifier.
2. Related Background Arts
An optical amplifier optically amplifies signal light transmitted through an optical transmission line such as an optical fiber transmission line in an optical transmission system to compensate transmission loss in the optical transmission line. The optical amplifier installed on an optical transmission line is equipped with an optical waveguide path such as an optical fiber for optical amplification and an exciting means for supplying pump light to the optical waveguide path for optical amplification. When signal light is input from the upstream side of the optical transmission line to the optical wave guide path for optical amplification to which pump light is supplied, the input signal light is optically amplified through the optical waveguide path for optical amplification and is output to the downstream. Such an optical amplifier includes a rare-earth element doped fiber amplifier, wherein an optical fiber to which rare-earth element is added is used as the optical waveguide path for optical amplification. For example, there is an erbium-doped fiber amplifier (EDFA) using, as an optical waveguide for optical amplification, an erbium-doped fiber (EDF) in which erbium (Er) is doped. An optical amplifier is modularized to be provided in a repeater station of an optical transmission system.
An EDFA can optically amplify signal light in the 1.55 xcexcm wavelength band (C-band), which is the spectrum band that exhibits the lowest loss of an optical fiber, and also can simultaneously amplify signal light of multi-wavelengths within an amplification spectrum band. Therefore, such an optical fiber amplifier is widely used as an optical amplifier applied to a wavelength division multiplexing (WDM) transmission system. Demands for such amplifiers are expected to increase further in the future.
In the above-mentioned structure where the 1.55 xcexcm wavelength band is the amplification spectrum band for the optical fiber amplifier, the fiber length needed for optical amplification is about 10 to 30 m. Thus, desired gain can be attained with a comparatively short fiber length. The dispersion in the optical fiber for optical amplification itself is hardly a problem because not only is the wavelength range of signal light comparatively narrow, but also there is a dispersion compensating fiber which exhibits zero dispersion in the vicinity of the 1.55 xcexcm wavelength. In order to meet the desire to suppress even low dispersion to a possible minimum level so as to speed up optical communication, an EDF having a zero dispersion wavelength in the 1.55 xcexcm wavelength was examined (See the reference literature: The Institute of Electronics, Information and Communication Engineers, General Meeting C-330 (1996)).
On the other hand, the demands for an optical amplifier using the 1.58 xcexcm wavelength band in which flat gain is possible in a wide wavelength range have increased rapidly in recent years. If an EDF used for optical amplification in the 1.55 xcexcm wavelength band is employed in an optical fiber amplifier for the 1.58 xcexcm wavelength band, the fiber length needed will be as long as 100 m to 120 m. In the case of using a specially designed EDF for optical amplification in the 1.58 xcexcm wavelength band (See the reference literature: OECC ""99, pp1356-1357 (1999)), the required fiber length will be about 50 m to 60 m.
The required fiber length can be shortened to some extent by increasing the concentration of Er added to an EDF, but there is a limit. Therefore, the dispersion characteristics of an EDF itself become a problem in the case of optical amplification in the 1.58 xcexcm wavelength band because dispersion is accumulated in the optical amplifier by signal light being transmitted in a long length EDF.
In order to overcome such problem, a structure is considered such that a device for dispersion compensation is connected to an EDFA so that dispersion in an optical transmission system as a whole may be compensated. In such a structure, however, there occur problems, such as connection loss and the deterioration of the noise figure, which are caused by connecting such dispersion-compensating device to an optical fiber. This becomes disadvantageous particularly in terms of increasing the number of wavelengths and the transmission speed in a WDM transmission system.
Japanese Patent Application Laid-Open No. 10-39155 and Japanese Patent Application Laid-Open No. 11-237520 disclose inventions in which optical amplification and dispersion compensation are performed simultaneously by adding a rare-earth element at a low concentration to a dispersion compensating fiber (DCF). However, in such a structure, it is difficult to make the required lengths of the respective fibers to coincide with each other. Because it is impossible to choose such length of fiber as to decrease the wavelength dependence of gain of the optical amplifier, a gain-flattening filter becomes necessary, and designing such a gain equalization filter is difficult. Also, in a dispersion compensating fiber, because xcex94n (the relative refractive index difference) of the core is great and the effective area (Aeff) is small, four-wave mixing (FWM) and cross-phase modulation (XPM) tend to occur easily. When these phenomena occur, thereby noise light is generated, and the waveform of signal light is degraded.
An object of the present invention is to provide an optical transmission system as well as an optical amplifier in which dispersion in the optical waveguide paths for optical amplification is easily and sufficiently compensated, and at the same time the waveform degradation of signal light is restrained.
In order to achieve this object, the optical amplifier is equipped with at least two optical waveguide paths connected in series which are doped with a fluorescent material to optically amplify signal light by pump light and which have dispersion different from each other in sign. The optical amplifier is also equipped with one or more exciting means that supply pump light to each of the optical waveguide paths described above.
In one embodiment of the present invention, overall chromatic dispersion in the whole optical amplifier may be zero at least in one wavelength of the spectrum band in which the optical amplifier has gain, and at least part of the spectrum band in which the optical amplifier has gain may be included in the L band. Also, at least one of the optical waveguide paths may be an EDF having dispersion equal to or less than xe2x88x9210 psxc2x7nmxe2x88x921xc2x7kmxe2x88x921 at the wavelength of 1580 nm, or an EDF having dispersion equal to or more than 5 psxc2x7nmxe2x88x921xc2x7kmxe2x88x921 at the wavelength of 1580 nm. In the present invention, the wavelength range of 1570 nm to 1605 nm is referred to as xe2x80x9cL bandxe2x80x9d.
Also, the optical transmission system is equipped with at least two optical waveguide paths connected in series which are doped with a fluorescent material to optically amplify signal light by pump light and which have dispersion different from each other in sign. The optical transmission system is also equipped with one or more exciting means that supply pump light to each of the optical waveguide paths described above.
The above and further objects and novel features of the invention will be more fully clarified from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.